1. Field of the Invention
The present invention relates to a terminal fitting including a gold plating layer in a terminal contact portion.
2. Description of the Related Art
A terminal fitting used in a connector has a structure including a terminal contact portion to be held in contact with a mating terminal and a wire crimping portion to be crimped and connected to a core of an insulated wire, and a copper alloy or brass material is generally used as a base material. In such a terminal fitting, a thin gold plating layer is locally formed in the terminal contact portion to improve electrical contact reliability with the mating terminal.
In the case of gold-plating a terminal made of a copper alloy or brass material, it is a technical common sense to form a nickel plating layer on a base material beforehand and form a gold plating layer thereon as disclosed, for example, in Japanese Unexamined Patent Publication No. H07-73769 below to prevent diffusion of gold atoms into the base material.
On the other hand, in a terminal fitting to be crimped and connected to a wire as described above, a relatively soft tin plating layer is formed in a wire crimping portion to improve adhesion between a core of the wire and a base material of the terminal fitting, thus an electrical contact property. Then, in the case of the above terminal fitting with the gold plating layer, metal layers are laminated in the order of the base material, the nickel plating layer and the gold plating layer in the terminal contact portion and in the order of the base material, the nickel plating layer, the tin plating layer and the gold plating layer in the wire crimping portion.
A terminal fitting with a wire of this type, particularly the one for vehicle may be used in an atmosphere with a severe thermal environment such as in an engine compartment. In recent years, attention has been paid to such a phenomenon that, if a connector is used under such an atmosphere, contact resistance between a terminal fitting and a wire gradually increases and there has been a demand for a countermeasure. Conventionally, attempts have been made to solve such a phenomenon by increasing a crimping force or forming serration grooves on the wire crimping portion, assuming that this phenomenon results from a problem in contact interfaces between the wire and the terminal fitting surface.
However, a study of the present inventors could find out not only a problem in the contact interfaces between the wire and the terminal fitting, but also a large problem in the construction of the metal layers in the wire crimping portion. Specifically, in the wire crimping portion of the above terminal fitting, the metals are laminated in the order of the base material, the nickel plating layer and the tin plating layer. The tin plating layer is relatively soft and originally has good adhesion to the core. However, since the nickel plating layer is present under the tin plating layer, alloying of the tin plating layer and the nickel plating layer is promoted and a nickel atomic ratio on the surface gradually increases under a high-temperature environment. Then, properties of the tin plating layer such as softness and low specific resistance are lost, and the contact interfaces may finely move due to thermal expansion/contraction and contact resistance may gradually increase upon being subjected to a thermal cycle. This has been a mechanism of increasing the contact resistance of the conventional gold-plated terminal fitting with the wire.